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The month of April is special for birders along the Gulf coast because they enjoy large, often sudden influxes of migrant birds: wood-warblers, orioles, grosbeaks, buntings, flycatchers, thrushes, cuckoos, and even hummingbirds. Places like High Island in Texas may be suddenly inundated with hundreds of birds of many species in the course of an afternoon — an event birders call a fallout.
Indeed, the migrant birds are often seen flying in directly from offshore, from the Gulf of Mexico. The obvious conclusion is that the birds have flown from their wintering areas in Central and South America, staging on the Yucatan Peninsula and then flying directly across the Gulf, a journey of about 600 miles that is, of necessity, accomplished nonstop.
Amazing, yes; so amazing that when the phenomenon was first reported years ago, many ornithologists failed to believe it, insisting that migrants must fly north via Mexico or up through Florida. Some do. But subsequent radar studies abundantly confirmed that the influx of birds along the Gulf coast is indeed the result of thousands having crossed the Gulf of Mexico, a risky flight fraught with the potential for fatal headwinds and storm fronts. Some birds barely make it, landing exhausted on the beaches. Some less fortunate wash up in the waves, having perished at sea. Most begin feeding frantically in the patches of woodland that dot the coast, essential stopover sites for fuel-diminished migrants.
Migration has a high cost measured in bird lives. The spectacular fallouts provide a thrill for birders but not for the birds themselves. The birds are there out of desperation. On a good flight across the Gulf, one with assisting tailwinds, migrants do not “fall out” along the coast but instead fly inland to settle and refuel in the rich streamside gallery forests abundant with emerging insects that lie farther inland.
Bird migration in all its various forms is spectacular, a real tour de force of nature and adaptation. Using celestial and solar cues for orientation, sensing latitudinal variation in Earth’s magnetic fields, flying nonstop for multiple hours, somehow knowing which tiny patch of habitat in a broad continent they are aiming for, birds set the bar high when it comes to remarkable adaptations. Why do they do it? What advantage does migration provide to offset the considerable risks?
Why birds migrate
The obvious outcome of spring migration is breeding. That is all the birds do once they arrive, why we call the destination, wherever it may be, the breeding grounds. Long-distance migrant birds fly north specifically to breed. Everybody knows that. But that begs the question: Why?
Maybe the birds are acting in accordance with deep historical roots. We often think their ancestors evolved in the north and developed migration to move south to avoid the cold of winter. Actually just the opposite may be the case for the vast majority of species. Many of our North American migrant species are genetically most closely related to tropical resident species. Perhaps they did not evolve to go south, but rather most had their origins in the equatorial regions and evolved to migrate north.
Consider the thrushes in the genus Catharus. One would think, looking at the similar plumages of the Hermit, Swainson’s, Gray-cheeked, Bicknell’s, and Veery, that these five species are each other’s closest genetic relatives. But that is so only in the case of Bicknell’s and Gray-cheeked. Looking at molecular data and genetic sequencing, the nearest relative of Swainson’s Thrush appears to be Black-billed Nightingale-Thrush, which is found in Costa Rica and western Panama. Ancestors of Veery, Gray-cheeked, and Bicknell’s Thrush appear to have split from Ruddy-capped Nightingale-Thrush approximately a million years ago, so the three species are recently evolved. The Ruddy-capped occurs in highland areas from central Mexico to western Panama.
Much of northern migration may have evolved from bird populations that were and still are tropical in distribution. The question that immediately arises is why leave the warm and wet tropics for a long and risky flight to the temperate zone, sometimes, as in the case of Gray-cheeked Thrush and Blackpoll Warbler, as far north as tree-line, inside the Arctic Circle?
As glacial recession began just over 20,000 years ago, the ice that had gripped the Northern Hemisphere yielded to increasingly temperate woodlands of conifers and deciduous broad-leaved trees, shaping the distribution of forests into the patterns we know today. Forests reclaimed much of North America. The world warmed, and the bugs appeared, lots of them. Temperate springs were flush with emerging insects, little packages of protein and fat that could feed myriad numbers of baby birds. This ecological opportunity took an evolutionary pathway when numerous bird populations began annual range expansions northward in spring, only to be forced to return to equatorial regions in autumn, as cold approached and arthropod food became greatly diminished. Migration, though steeped with risk, evolved because it paid dividends to the migrants. The benefits outweighed the costs. What might these benefits be?
Rich and abundant food
First and foremost, if birds time their arrival in the north with the spring insect emergence, they have a ready abundance of nutritious food to feed their young, young that are exposed in open cup nests and constantly at risk from predators. Birds may make do with relatively small territories that, with insect abundance, ensure they have sufficient access to food for themselves and their nestlings. With rich and abundant food, the nestlings, most of which hatch with no feathers, will grow faster and fledge sooner, reducing predator risk.
Also, spring and summer days are longer than in the tropics, where, on the equator, there are 12 hours of light and 12 hours of darkness. Given more hours of daylight, temperate breeders can continue to find food and feed young. Longer days mean more continuous nest provisioning, helping speed the growth of nestlings. Abundant food and longer days also equate to larger clutch sizes.
Consider the familiar Baltimore Oriole. Its average clutch size is four to five eggs. The similar Yellow-tailed Oriole, a permanent resident of lowlands and dense thickets from southern Mexico through much of northern South America, has a clutch of three eggs, which is on the high side for tropical passerines. A Baltimore Oriole will fly thousands of miles in its lifetime in order to achieve a larger clutch size than if it remained sedentary in the tropics. A Yellow-tailed Oriole will fly but a few hundred miles in its lifetime, all local trips. Most tropical songbird species are highly sedentary, inhabiting the same small area of forest or thicket for their lifetimes. They know their turf and do not leave it.
Birds face an obvious trade-off between the costs and benefits of long-distance migration and remaining resident in the tropics. At first glance, it would seem that migrants must “win.” They lay more eggs per season and raise more young. But as a tired and injured Indiana Jones said to Marion in “Raiders of the Lost Ark,” “It’s not the years, honey, it’s the mileage.” Mileage adds up to risk, and risk eventually means death. The reality is that temperate-zone songbirds, migrants and residents alike, generally have significantly shorter lifespans than birds in the more climatically equitable equatorial tropics.
Climate, with its extreme seasonal variability, has much to do with this reality, but migrants compound the risk of death by the very act of migration. The Red-eyed Vireo singing in your shaded woods is lucky if it lives five years. The oldest on record was 10 years old. The average annual survivorship of adults is only about 60 percent.
Recent research on migrant and resident populations of Dark-eyed Juncos has added evidence that mileage matters in how birds age. By measuring the lengths of telomeres, which are at the tips of chromosomes, Carolyn Bauer of North Dakota State University and colleagues showed that migrant birds have shorter telomeres. Why is this of interest? Telomeres are protective of chromosomes, but they are subject to a degenerative process termed oxidative stress, which correlates with the aging process.
Bauer and her colleagues concluded that migrant juncos spend less energy on self-maintenance in order to accommodate the stresses imposed by migration and, as evidenced in telomere shortening, the migrant birds literally age more rapidly than resident birds. Long-distance migrants such as wood-warblers may indeed be destined to live shorter lives because they migrate.
In contrast, the White-bearded Manakin that you might observe on its lek in Trinidad could easily survive 15 or more years, as was confirmed many years ago by banding studies conducted by the eminent ornithologist David Snow. Red-eyed Vireo has an average clutch size of four eggs, and the manakin’s clutch size is but two eggs. Many vireo nests fail due to predation. Survivorship of fledged birds is low. But even more manakin nests fail due to predation, which appears to be uniquely high for virtually all bird species in the predator-saturated tropics.
Keep in mind, however, that in both the case of the vireo and the manakin, only two birds need to survive to adulthood to potentially maintain a stable population (one to replace the male, one to replace the female). A White-bearded Manakin has many more years to attempt reproduction than a Red-eyed Vireo. That’s the trade-off. In terms of what ornithologists refer to as lifetime reproductive success, the two species may well be about equal. The same applies to Baltimore and Yellow-tailed Orioles. A Yellow-tailed need not equal its northern-breeding relative in clutch size. It will likely live longer and breed more often over the course of that lifetime, thus compensating for its smaller annual clutch size.
Small clutch size is the rule in the tropics, likely in part because food is more limited (there is no spring insect flush or long days of summer), and predation, as already mentioned, is uniquely high. Anything from monkeys, kinkajous, opossums, snakes, toucans, various insects, and spiders to forest falcons routinely prey on eggs and nestlings (and sometimes adult birds) in the tropics. The longer it takes to raise young birds, the more likely the risk that the nest will fall to a predator. But laying fewer eggs means less time provisioning and thus less time for the nesting cycle, increasing ever so slightly the chance of success. Though most nests in the tropics fail, repeated attempts to nest, provided by greater longevity, compensate for the huge losses.
Legendary ornithologist Alexander Skutch first showed that diverse species such as flycatchers (Tyrannidae) that occupy numerous kinds of tropical habitats essentially all have a clutch size of two eggs. Only in the temperate zone does tyrannid clutch size increase. The widespread and well-named Tropical Kingbird has a usual clutch size of two eggs, occasionally three. The highly similar Western Kingbird lays between three and five eggs, sometimes as many as seven. But again, Tropical Kingbirds enjoy a greater average longevity than their northern cousins.
Ornithologists have only recently begun to appreciate the full extent of differences in life-history characteristics between temperate and tropical passerines. Differences go beyond clutch size and longevity to include pair bonding between the sexes and extra-pair matings that are now recognized to be common in temperate-zone nesting birds and far less so in the tropics.
Compressed vs. relaxed breeding seasons
Here is another trade-off. In the temperate zone, the breeding season is compressed and synchronous. In a period of a couple of months, all American Redstarts, Wood Thrushes, and Scarlet Tanagers are simultaneously breeding. Opportunities for males and females to mate outside their pair bond are numerous and constant. Pairs hear and see each other. Though socially monogamous, these species and many others are, to a significant degree, genetically polygamous. This means that a clutch of Scarlet Tanager eggs may have several fathers, though only one male (socially bonded with the female bird) will provision the nestlings.
Male birds often try and guard females to reduce instances of extra-pair matings, adding stress to their lives. Such occurrences are much less common among tropical resident bird species, because their breeding season is far more “relaxed,” more protracted. Some members of a species are breeding while some are not. Extra-pair matings add an additional stress to temperate-zone breeders that is not nearly as present among tropical bird species, and that also likely contributes in a complex manner to lifetime reproductive success and overall longevity. In Costa Rica and Panama, Tropical Kingbirds breed from March through July, a five-month period. A Thick-billed Euphonia may nest anytime from March through September. A pair of Dusky Antbirds could breed anytime from February through September, an eight-month breeding season. The key point here is that Dusky Antbirds do not breed in synchrony, as temperate-zone birds are forced to do by the realities of seasonal change. So opportunities for extra-pair matings are greatly reduced in the tropics, as are stress levels that accompany such behavior.
At first glance, it may seem that long-distance migrant species such as Blackburnian Warbler and Veery have paid a huge evolutionary cost, one that does not burden their tropical stay-at-home relatives. They embark annually on long and risky migrations, endure a stressful high-intensity breeding season, and ultimately live short lives. But population sizes of temperate-zone long-distance migrants are large, quite possibly larger than populations of scores of tropical species, many of which have restricted distributions. That is the measure of evolutionary success. The trade-off may not be pretty, but it works.
John Kricher is professor of biology at Wheaton College and past president of the Wilson Ornithological Society, Association of Field Ornithologists, and Nuttall Ornithological Club. He is the author of The New Neotropical Companion, Galápagos: A Natural History, and other books. He wrote about Blackpoll Warbler in our October 2017 issue.